Negatively chargeable toner, method for producing the same, and full color image forming apparatus using the negatively chargeable toner

ABSTRACT

A negatively chargeable toner containing: resin particles containing a colorant; and an external additive which coats the surface of the resin particles, wherein the external additive contains; first silica particles having a number mean primary particle size of 5 to 20 nm and second silica particles having a number mean primary particle size of 30 to 50 nm; surface-modified silica particles which are surface modified by wet treatment using an oxide or hydroxide of at least one metal selected from the group consisting of titanium, tin, zirconium and aluminum, and are further subjected to hydrophobic treatment; and aluminum oxide-silicon dioxide composite oxide particles obtained by flame hydrolysis and hydrophobic treatment.

FIELD OF THE INVENTION

The present invention relates to a negatively chargeable toner used forelectrophotography, electrostatic printing and the like, a method forproducing the same, and a image forming apparatus using the negativelychargeable toner.

BACKGROUND OF THE INVENTION

In electrophotography, an electrostatic latent image formed on aphotoreceptor provided with a photoconductive material is developed withcolorant-containing toner particles, and then, the resulting toner imageis fixed onto a transfer material such as paper by heat, pressure or thelike, thereby forming a copy or printed matter.

In order to improve physical properties of the toner, it is generallycarried out to add an external additive (surface treating agent) to thetoner particles. However, the toner particles have a size distribution,so that even when the external additive is added to the toner particles,there is a distribution in charging amount. Accordingly, even anegatively chargeable toner inevitably contains positively charged tonerparticles. In an image forming apparatus preparing an image by negativecharge reversal development, there is the problem that the toner adheresto a non-image area of an image carrier (photoreceptor), the amount ofthe toner adhered to cause fogging further increases with an increase inthe number of printed sheets, correlatively with deterioration of thetoner, and the load of cleaning in the photoreceptor also increases. Inparticular, when the toner is used as a full color toner, a selectivedevelopment occurs by a broadened charge distribution of the tonerderived from the size distribution of the toner particles, or whenpositively charged toner particles exist, not only the problem offogging but also the problem of the development of a “reverse transfertoner” reversely transferred to the photoreceptor in color superpositionis encountered.

In a single-component non-magnetic toner, when a large amount of silicaparticles are added in order to prevent deterioration of the toner, thefluidity of the toner is secured, but the fixability is reduced.Further, in order to enhance the negatively chargeable capacity of thetoner, it is known to add positively chargeable titania having apolarity opposite to that of the toner together with silica particles.However, positively chargeable titania is liberated from surfaces of thetoner particles in some cases, with an increase in the number of printedsheets, which poses the problem with respect to the maintenance of imagedensity in continuous printing. Further, when the negatively chargeablecapacity of the toner becomes too high, the printed image density isreduced. To prevent this, it is also known to use titania having arelatively low electric resistance and a large primary particle size forpreventing the titania particles from being embedded. However, anincrease in the number of printed sheets raises the problem that titaniais librated from surfaces of toner mother particles, resulting infailure to sufficiently exhibit the effects. Further, in order toprevent filming, it is also known to add aluminum oxide (alumina)particles together with silica particles. However, it is difficult tosubject to hydrophobic treatment the alumina particles, and theintroduction of excessive alumina particles raises the problem ofshortening the life of a photoreceptor.

Further, when various metal oxide particles are externally added, it isknown to adding particles having a relatively large particle size andparticles having a relatively small particle size in combination,thereby improving the fluidity, cleaning property, transfercharacteristics, charge property and “durability” (prolonging the lifeof the toner) of the toner. Furthermore, it is also known to specify theliberating rate of an external additive from toner mother particles,thereby making possible the charge stabilization, antifogging, whitespot prevention, maintenance of image density and filming prevention. Inaddition, it is known that combined oxide particles comprising aluminumoxide and silicon dioxide are used as an external additive, therebybeing able to obtain toner particles excellent in fluidity and showingstable charge behavior (reference 1). However, in any case, when thetoner is applied to a full color toner, a selective development occursby a broadened charge distribution of the toner derived from the sizedistribution of the toner particles, or the existence of positivelycharged toner particles poses not only the problem of fogging but alsothe problem of the development of a “reverse transfer toner” reverselytransferred to the photoreceptor in color superposition.

Further, it is known to modify surfaces of single fine particles orparts thereof with another material, instead of using plural kinds ofexternal additives in combination in toner particles, to obtain theeffects of the plural kinds of external additives, such as the fluidity,charge property, maintenance of image density and fogging or filmingprevention. For example, a toner for electrophotography is known inwhich hydrophobic fine particles obtained by coating fine silicaparticles with a hydroxide or an oxide of one or more of titanium, tin,zirconium and aluminum in an aqueous system, and further coatingsurfaces thereof with an alkoxysilane are used as an external additive(reference 2). Further, a toner is proposed in which silica-containingtitanium oxide particles obtained by coating outer surfaces of silicaparticles with titanium oxide are used as an external additive(reference 3). According to the addition of such particles, it ispossible to decrease the amount of positively charged toner particles,but the average charging amount is insufficient. Accordingly, neither ofthem achieves both objects of improving the transfer efficiency andreducing the reverse transfer toner.

[Reference 1]

-   -   JP-A-2000-181130 (the term “JP-A” as used herein means an        “unexamined published Japanese patent application”)

[Reference 2]

-   -   JP-A-2002-29730

[Reference 3]

-   -   JP-A-2002-148848

[Reference 4]

-   -   Japanese Patent No. 2533067

[Reference 5]

-   -   JP-A-2001-300083

The present invention relates to a negatively chargeable toner having anovel combination of external additives, which provides little foggingin a non-image area caused by a toner on a photoreceptor in developmentand can prevent excessive charge, resulting in improvement in thetransfer efficiency and the prevention of the development of a reversetransfer toner on the photoreceptor in superposition of the second coloror later, a method for producing the same, and a full color imageforming apparatus using the negatively chargeable toner.

SUMMARY OF THE INVENTION

The present inventors have made eager investigation to examine theproblem. As a result, it has been found that the foregoing objects canbe achieved by the following negatively chargeable toner, the method ofproducing the same and the full color image forming apparatus using thenegatively chargeable toner. With this finding, the present invention isaccomplished.

The present invention is mainly directed to the following items:

-   -   1. A negatively chargeable toner comprising: resin particles        comprising a colorant; and an external additive which coats the        surface of the resin particles, wherein the external additive        comprises; first silica particles having a number mean primary        particle size of 5 to 20 nm and second silica particles having a        number mean primary particle size of 30 to 50 nm;        surface-modified silica particles which are surface modified by        wet treatment using an oxide or hydroxide of at least one metal        selected from the group consisting of titanium, tin, zirconium        and aluminum, and are further subjected to hydrophobic        treatment; and aluminum oxide-silicon dioxide composite oxide        particles obtained by flame hydrolysis and hydrophobic        treatment.    -   2. The negatively chargeable toner according to item 1, wherein        the total amount of the first and second silica particles are        0.5 to 1.5% by weight based on the weight of the resin        particles, wherein the weight ratio of the first silica        particles to the second silica particles is from 5/1 to 1/5.    -   3. The negatively chargeable toner according to item 1, wherein        the amount of the surface-modified silica particles is 0.005 to        0.5% by weight based on the weight of the resin particles,        wherein the amount of the aluminum oxide-silicon dioxide        composite oxide particles is 0.005 to 0.5% by weight based on        the weight of the resin particles, wherein the total amount of        the surface-modified silica particles and the aluminum        oxide-silicon dioxide composite oxide particles is from 0.01 to        1% by weight based on the weight of the resin particles.    -   4. The negatively chargeable toner according to item 1, wherein        the amount of the external additive is 0.51 to 2.5% by weight        based on the weight of the resin particles.    -   5. The negatively chargeable toner according to item 1, which is        produced through a polymerization method.    -   6. The negatively chargeable toner according to item 1, wherein        the negatively chargeable toner has a sphericity of 0.94 or        more.    -   7. The negatively chargeable toner according to item 1, wherein        the negatively chargeable toner has a number mean particle size        of 9 mm or less.    -   8. A full color toner comprising the negatively chargeable toner        according to item 1.    -   9. A process for producing a negatively chargeable toner        comprising the steps of: adding, to resin particles comprising a        colorant, first silica particles having a number mean primary        particle size of 5 to 20 nm and second silica particles having a        number mean primary particle size of 30 to 50 nm; and further        adding thereto surface-modified silica particles which are        surface modified by wet treatment using an oxide or hydroxide of        at least one metal selected from the group consisting of        titanium, tin, zirconium and aluminum, and are further subjected        to hydrophobic treatment, and aluminum oxide-silicon dioxide        composite oxide particles obtained by flame hydrolysis and        hydrophobic treatment.    -   10. A full color image forming apparatus comprising: a        photoreceptor on which toner images are to be formed; an        intermediate transfer medium for transferring the toner images        formed on the photoreceptor to a recording medium; and a toner        for forming the toner images comprising: a negatively chargeable        toner comprising: resin particles comprising a colorant; and an        external additive which coats the surface of the resin        particles, wherein the external additive comprises; first silica        particles having a number mean primary particle size of 5 to 20        nm and second silica particles having a number mean primary        particle size of 30 to 50 nm; surface-modified silica particles        which are surface modified by wet treatment using an oxide or        hydroxide of at least one metal selected from the group        consisting of titanium, tin, zirconium and aluminum, and are        further subjected to hydrophobic treatment; and aluminum        oxide-silicon dioxide composite oxide particles obtained by        flame hydrolysis and hydrophobic treatment.    -   11. The full color image forming apparatus according to item 10,        wherein the photoreceptor is a negatively chargeable organic        photoreceptor.    -   12. The full color image forming apparatus according to item 10,        wherein the intermediate transfer medium is a belt.    -   13. The full color image forming apparatus according to item 10,        further comprising a image developing device, wherein the        photoreceptor and the image developing device are integrated to        form a process cartridge, wherein the process cartridge is        detachably mounted on the image forming apparatus.    -   14. The full color image forming apparatus according to item 10,        wherein the ratio of peripheral velocity of the photoreceptor to        the intermediate transfer medium is from 0.95 to 1.05.    -   15. The negatively chargeable toner according to item 1, wherein        the first and second silica particles are subjected to        hydrophobic treatment.    -   16. The process for producing a negatively chargeable toner        according to item 9, wherein the first and second silica        particles are subjected to hydrophobic treatment.    -   17. The full color image forming apparatus according to item 10,        wherein the first and second silica particles are subjected to        hydrophobic treatment.    -   18. The negatively chargeable toner according to item 1, wherein        the external additive further comprising a metal soap particle.    -   19. The process for producing a negatively chargeable toner        according to item 9, further comprising a step of adding a metal        soap particle to the resin particles.    -   20. The full color image forming apparatus according to item 10,        wherein the external additive further comprising a metal soap        particle.    -   21. The negatively chargeable toner according to item 3, wherein        the weight ratio of the surface-modified silica particles to the        aluminum oxide-silicon dioxide composite oxide particles is from        2/50 to 50/2.    -   22. The full color image forming apparatus according to item 10,        wherein the work function of the negatively chargeable toner is        higher than the work function of the surface of the        photoreceptor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view illustrating an embodiment of a contact developingprocess in an image forming apparatus using a toner of the presentinvention.

FIG. 2 is a view illustrating an embodiment of a non-contact developingprocess in an image forming apparatus using a toner of the presentinvention.

FIG. 3 is a view illustrating an embodiment of a full color printer of a4-cycle system using a toner of the present invention.

FIG. 4 is a schematic front view illustrating an embodiment of a fullcolor printer of a tandem system using a toner of the present invention.

FIG. 5 is a view showing a burner apparatus for producing combined oxideparticles in the present invention.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, it has been discovered that in anegatively chargeable toner comprising: resin particles comprising acolorant; and an external additive which coats the surface of the resinparticles, wherein the external additive contains; (1) first silicaparticles having a number mean primary particle size of 5 to 20 nm andsecond silica particles having a number mean primary particle size of 30to 50 nm (hereinafter also referred to as silica particles different inparticle size); (2) surface-modified silica particles which are surfacemodified by wet treatment using an oxide or hydroxide of at least onemetal selected from the group consisting of titanium, tin, zirconium andaluminum, and are further subjected to hydrophobic treatment(hereinafter also referred to as surface-modified silica particles); and(3) aluminum oxide-silicon dioxide composite oxide particles obtained byflame hydrolysis and hydrophobic treatment (hereinafter also referred toas aluminum oxide-silicon dioxide composite oxide particles or combinedoxide particles) are contained in specified amounts, respectively, asthe external additive, thereby enhancing the charge characteristics ofthe toner and decreasing the amount of reversely charged tonerparticles, which makes it possible to stabilize the chargecharacteristics of the toner and to improve the transfer efficiency,allowing the formation of the negatively chargeable toner suitable as afull color toner.

As the toner mother particles, there are exemplified toner particlesobtained by a pulverization method or a polymerization method. As forthe toner obtained by the pulverization method, a release agent and acharge control agent are added to a binder resin containing at least apigment, followed by uniform mixing by a Henschel mixer. Then, theresulting mixture is melt kneaded by a twin-screw extruder. Aftercooling, the melt is roughly pulverized and finely pulverized, and theresulting particles are classified. Further, external additive particlesare adhered thereto to form toner particles.

As the binder resin, a synthetic resin used as a resin for toner isavailable. Examples thereof include homopolymers or copolymerscontaining styrene or styrene substituents, styrenic resins such aspolystyrene, poly-α-methylstyrene, chloropolystyrene, astyrene-chlorostyrene copolymer, a styrene-propylene copolymer, astyrene-butadiene copolymer, a styrene-vinyl chloride copolymer, astyrene-vinyl acetate copolymer, a styrene-maleic acid copolymer, astyrene-acrylate copolymer, a styrene-methacrylate copolymer, astyrene-acrylate-methacrylate copolymer, a styrene-methylα-chloroacrylate copolymer, a styrene-acrylonitrile-acrylate copolymerand a styrene-vinyl methyl ether copolymer, polyester resins, epoxyresins, urethane-modified epoxy resins, silicone-modified epoxy resins,vinyl chloride resins, rosin-modified maleic acid resins, phenyl resins,polyethylene, polypropylene, ionomer resins, polyurethane resins,silicone resins, ketone resins, ethylene-ethyl acrylate copolymers,xylene resins, polyvinyl butyral resins, terpene resins, phenol resinsand aliphatic or alicyclic hydrocarbon resins. They can be used eitheralone or in combination. In the present invention, styrene-acrylateresins, styrene-methacrylate resins and polyester resins are preferred.The binder resin preferably has a glass transition temperature rangingfrom 50 to 75° C., and a flow softening temperature ranging from 100 to150° C.

As the colorant, there is available a colorant for toner in which dyesand pigments of yellow, magenta, cyan and black are used either alone orin combination, and a toner having at least 4 colors is used.

Examples of colorants for black (K) include carbon black, lamp black,magnetite and titanium black.

Examples of colorants for yellow (Y) include Chrome Yellow, Hansa YellowG, Quinoline Yellow, C.I. Pigment Yellow 12, C.I. Pigment Yellow 17,C.I. Pigment Yellow 97, C.I. Pigment Yellow 180, C.I. Solvent Yellow 162and Benzidine Yellow.

Examples of colorants for magenta (M) include Quinacridone, C.I. PigmentRed 48:1, C.I. Pigment Red 122, C.I. Pigment Red 57:1, C.I. Pigment Red184 and Rhodamine 6G.

Further, examples of colorants for cyan (C) include ultramarine blue,Aniline Blue, Phthalocyanine Blue, Phthalocyanine Green, Chalco OilBlue, Rose Bengal, Malachite Green Lake, C.I. Pigment Blue 5:1 and C.I.Pigment Blue 15:3.

As the release agent, there is available a release agent for toner.Examples thereof include paraffin wax, micro wax, microcrystalline wax,candelilla wax, carnauba wax, ester wax, rice wax, montan wax,polyethylene wax, polypropylene wax, oxidized polyethylene wax andoxidized polypropylene wax. Polyethylene wax, polypropylene wax,carnauba wax, ester wax and rice wax are preferably used among others.

As the charge control agent, there is available a charge control agentfor toner. Examples thereof include oil black, Oil Black BY, BontronS-22 and S-34 (manufactured by Orient Chemical Industries, Ltd.), metalcomplexes of salicylic acid E-81 and E-84 (manufactured by OrientChemical Industries, Ltd.), a thioindigo-based pigment, a sulfonylaminederivative of copper phthalocyanine, Spilon Black TRH (manufactured byHodogaya Chemical Co., Ltd.), a calixarene-based compound, an organicboron compound, a fluorine-containing quaternary ammonium salt-basedcompound, a monoazo metal complex, an aromatic hydroxycarboxylicacid-based metal complex, an aromatic dicarboxylic acid-based metalcomplex and a polysaccharide. For a color toner, a colorless or whiteagent is preferred among others.

As for the ratio of components in the toner obtained by thepulverization method, the amount of the colorant is preferably from 0.5to 15 parts by weight, and more preferably from 1 to 10 parts by weight,the amount of the release agent is preferably from 1 to 10 parts byweight, and more preferably from 2.5 to 8 parts by weight, and theamount of the charge control agent is preferably from 0.1 to 7 parts byweight, and more preferably from 0.5 to 5 parts by weight, based on 100parts by weight of the binder resin.

In the toner obtained by the pulverization method, in order to improvethe transfer efficiency, the toner particles are preferably subjected tospheroidization treatment. When an apparatus in which pulverization intorelatively round spherical particles is possible, for example, a turbomill (manufactured by Kawasaki Heavy Industries, Ltd.) known as amechanical pulverizer, is used in a pulverization process, thesphericity can be increased to 0.93. Further, the sphericity can beincreased to 1.00 by treating the pulverized toner using a hot airspheroidizing apparatus (manufactured by Nippon Pneumatic Mfg. Co.,Ltd.). In the present invention, the sphericity is preferably adjustedto 0.94 or more. When the sphericity is less than 0.94, the desiredtransfer efficiency is not obtained.

Then, the toner obtained by the polymerization method is obtained by asuspension polymerization method, an emulsion polymerization method or adispersion polymerization method, and can be suitable for a full colortoner. In the suspension polymerization, a monomer composition preparedby melting or dispersing a compound material of a polymerizable monomer,a coloring pigment and a release agent, and further, a dye, apolymerization initiator, a crosslinking agent, a charge control agentand other additives as needed is added into an aqueous phase containinga suspension stabilizer (a water-soluble polymer or a slightlywater-soluble inorganic material) with stirring, granulated andpolymerized, thereby being able to form colored polymerized tonerparticles having a desired particle size. Of the materials used in thepreparation of the toner obtained by the polymerization method, thecolorant, release agent and charge control agent may be the samematerials as used in the above-mentioned toner obtained by thepulverization method.

In the emulsion polymerization method, a monomer and a release agent,and further, a polymerization initiator, an emulsifier (surfactant) andthe like as needed are dispersed in water, and polymerization isconducted. Then, a colorant, a charge control agent and a coagulant(electrolyte) are added in a coagulation process, thereby being able toform colored toner particles having a desired particle size.

As the polymerizable monomer component, there is available a known vinylmonomer. Examples thereof include styrene, o-methylstyrene,m-methylstyrene, p-methylstyrene, α-methylstyrene, p-methoxystyrene,p-ethylstyrene, vinyltoluene, 2,4-dimethylstyrene, p-n-butylstyrene,p-phenylstyrene, p-chlorostyrene, divinylbenzene, methyl acrylate, ethylacrylate, propyl acrylate, n-butyl acrylate, isobutyl acrylate, n-octylacrylate, dodecyl acrylate, hydroxyethyl acrylate, 2-ethylhexylacrylate, phenyl acrylate, stearyl acrylate, 2-chloroethyl acrylate,methyl methacrylate, ethyl methacrylate, propyl methacrylate, n-butylmethacrylate, isobutyl methacrylate, n-octyl methacrylate, dodecylmethacrylate, hydroxyethyl methacrylate, 2-ethylhexyl methacrylate,stearyl methacrylate, phenyl methacrylate, acrylic acid, methacrylicacid, maleic acid, fumaric acid, cinnamic acid, ethylene glycol,propylene glycol, maleic anhydride, phthalic anhydride, ethylene,propylene, butylene, isobutylene, vinyl chloride, vinylidene chloride,vinyl bromide, vinyl fluoride, vinyl acetate, vinyl propionate,acrylonitrile, methacrylonitrile, vinyl methyl ether, vinyl ethyl ether,vinyl ketone, vinyl hexyl ketone and vinylnaphthalene. Asfluorine-containing monomers, there are available, for example,2,2,2-trifluoroethyl acrylate, 2,2,3,3-tetrafluoropropyl acrylate,vinylidene fluoride, ethylene trifluoride, tetrafluoroethylene andtrifluoropropylene, because the fluorine atoms are effective fornegative charge control.

The emulsifiers (surfactants) include, for example, sodiumdodecylbenzenesulfonate, sodium tetradecylsulfate, sodiumpentadecylsulfate, sodium octylsulfate, sodium oleate, sodium laurate,potassium stearate, calcium oleate, dodecylammonium chloride,dodecylammonium bromide, dodecyltrimethylammonium bromide,dodecylpyridinium chloride, hexadecyltrimethylammonium bromide, dodecylpolyoxyethylene ether, hexadecyl polyoxyethylene ether, laurylpolyoxyethylene ether and sorbitan monooleate polyoxyethylene ether.

The polymerization initiators include, for example, potassiumpersulfate, sodium persulfate, ammonium persulfate, hydrogen peroxide,4,4′-azobiscyanovaleric acid, t-butyl hydroperoxide, benzoyl peroxideand 2,2′-azobisisobutyronitrile.

The coagulants (electrolytes) include, for example, sodium chloride,potassium chloride, lithium chloride, magnesium chloride, calciumchloride, sodium sulfate, potassium sulfate, lithium sulfate, magnesiumsulfate, calcium sulfate, zinc sulfate, aluminum sulfate and ironsulfate.

As methods for controlling the sphericity of the toner obtained by thepolymerization method, in the emulsion polymerization method, thetemperature and time are adjusted in a coagulation process of secondaryparticles, thereby being able to freely change the sphericity. The rangethereof is from 0.94 to 1.00. Further, in the suspension polymerizationmethod, perfect spherical toner particles are obtainable, so that thesphericity ranges from 0.98 to 1.00. However, the sphericity can befreely controlled from 0.94 to 0.98 by deforming the toner particles byheating them at a temperature higher than the glass transitiontemperature (Tg) of the toner.

Further, for both the toner obtained by the pulverization method and thetoner obtained by the polymerization method, the number mean particlesize of the toner is preferably 9 μm or less, and more preferably from4.5 to 8 μm. Toner particles having a number mean particle size largerthan 9 μm are deteriorated in reproducibility of the resolution thereof,compared to toner particles having a smaller particle size, even when alatent image is formed at a high resolution of 1,200 dpi or more. On theother hand, when the particle size is smaller than 4.5 μm, opacifyingproperties by the toner are lowered, and the amount of an externaladditive added for enhancing fluidity increases. As a result, there isunfavorably a tendency to lower fixing performance. The above-mentionedmean particle size of the toner mother particles and toner particles inthe present invention is a value measured with a particle image analyzer(manufactured by Sysmex Corporation, FPIA2100), and means the numbermean particle size.

The external additives will be described below. The surfaces of thetoner mother particles are coated with the two kinds of silica particlesdifferent in mean primary particle size, the surface-modified silicaparticles and further the aluminum oxide-silicon dioxide composite oxideparticles as the external additives, in specified amounts, respectively.The particle size of the external additive in the present invention isobserved and measured by electron microscope, and means the number meanparticle size.

The silica particles are added in order to impart the negativelychargeable property and fluidity. In the present invention, the silicaparticles are used as a mixture of silica particles different in themean particle size distribution. Small-sized silica particles having amean primary particle size of 5 to 20 nm, preferably 7 to 16 nm are usedin combination with large-sized silica particles having a mean primaryparticle size of 30 to 50 nm, preferably 30 to 40 nm. The small-sizedparticles can provide the fluidity and negatively chargeable property,while the large-sized particles can prevent the external additiveparticles from being embedded in the toner mother particles. When themean particle size of the primary particles of the silica particles issmaller than 5 nm, the silica particles become liable to be embedded inthe mother particles of the toner, and become liable to be negativelycharged. On the other hand, exceeding 50 nm results in deterioration ofthe effect of imparting the fluidity to the toner mother particles,which makes it difficult to uniformly negatively charge the toner,resulting in the tendency to increase the amount of toner particlesreversely charged, i.e., positively charged.

The silica particles are added in an amount of preferably 0.5 to 1.5% byweight per toner mother particles. Less than 0.5% by weight results infailure to provide the effect of imparting the fluidity, whereasexceeding 1.5% by weight unfavorably causes deterioration of thefixability. Further, the ratio (weight ratio) of the small-sized silicaparticles to the large-sized silica particles is preferably from 5/1 to1/5. Too much the small-sized particles bring about deterioration of thefixability, whereas too little lead to a reduction in fluidity.

In the present invention, it is preferable that the silica particles aresubjected to hydrophobic treatment. In the present invention, either ofparticles prepared from a silicon halide by a dry process and particlesprecipitated from a silicon compound in a solution by a wet process canbe preferably used.

Then, the surface-modified silica particles are prepared by a methoddescribed in reference 2, and obtained by adding dropwise an aqueoussolution of an oxide or hydroxide of at least one metal selected fromtitanium, tin, zirconium and aluminum into an aqueous dispersion ofsilica particles produced by a wet process or a gas phase process andhaving a specific surface of 50 to 400 m²/g. As for the amount coated,the oxide or hydroxide of at least one metal selected from titanium,tin, zirconium and aluminum is applied in an amount of 1 to 30% byweight based on the silica particles.

The surface-modified silica particles are subjected to hydrophobictreatment by adding an alkoxysilane into a slurry in which thesurface-modified silica particles have been formed, generally in anamount of 30 to 50% by weight based on the fine silica particles,followed by filtration, washing and drying to preparehydrophobic-treated, surface-modified silica particles. Thehydrophobilization treatment may be conducted by adding an alkoxylsilaneand using a Henschel mixer after filtration, washing and drying of thesurface-modified silica particles. The number-based mean primaryparticle size of the hydrophobic-treated, surface-modified silicaparticles is preferably from 5 to 50 nm, and more preferably from 7 to40 nm.

The surface-modified silica particles are preferably added in an amountof 0.005 to 0.5% by weight, more preferably 0.08 to 0.5% by weight,based on the toner mother particles. The surface-modified silicaparticle has a negative frictional charge site based on the silicacomponent, and a positive frictional charge site based on the titanium,tin, zirconium or aluminum component, so that it has the function ofpreventing excessive charge caused by the silica particle. It istherefore conceivable that stable image formation can be performed.Further, the silica component constituting a base in thesurface-modified silica particle is fixedly adhered to a surface of thetoner through the silica particle. As a result, it is conceivable thatthe liberating rate of the external additives in continuous printing isdecreased, which makes it possible to impart stable chargecharacteristics for a long period of time. When the surface-modifiedsilica particles are added in an amount of more than 0.5% by weight, theproblem is encountered that the positive frictional charge sitesincrease to many. It is therefore unfavorable. In the following examplesand the like, descriptions are made for examples in which the silicaparticles are surface modified with the titanium compound. However, thesurface-modified silica particles surface modified with the tin,zirconium or aluminum compound also exhibit a similar action.

Then, the aluminum oxide-silicon dioxide composite oxide particles areprepared by a method for producing a fine aluminum oxide-silicon dioxidecomposite oxide particles, which is described in reference 4 andcomprises the following preparation steps:

-   -   (1) A silicon halide and an aluminum halide are evaporated, and        the respective vapors are homogeneously combined with air,        oxygen and hydrogen, together with a carrier gas, in a combining        unit;    -   (2) Then, the resulting combined vapor is supplied to a burner        to conduct flame hydrolysis reaction in a combustion chamber,        and the resulting gas and solid are cooled in a heat exchange        unit; and    -   (3) The gas is separated from the solid, and halide residues        adhering to the product are removed by heat treatment using wet        air to obtain the combined oxide particles.

The proportions of Al₂O₃ and SiO₂ are appropriately adjusted by reactionconditions such as the amounts of the silicon halide and aluminum halidesupplied, the amount of hydrogen supplied and the amount of the airsupplied. It is preferable that the content of Al₂O₃ is adjusted withinthe range of 55 to 85% by weight, and also preferable that the contentof SiO₂ is adjusted within the range of 15 to 45% by weight. Further,the combined oxide particles are granulated in the flame, therebyproviding particles having an amorphous structure, sufficient finegraininess and a specific surface area determined by the BET method of20 to 200 m²/g in general. The primary particle size of the combinedoxide particles is preferably from 7 to 80 nm, and more preferably from10 to 40 nm, and particles having a particle size of 20 nm or morepreferably occupy 30% or more based on the number.

It is preferable that the combined oxide particles are added in anamount of 0.005 to 0.5% by weight, more preferably 0.08 to 0.5% byweight, based on the toner mother particles.

It is conceivable that the combined oxide particles give two frictionalcharge sites of positive and negative when added to the toner motherparticles. However, it is conceivable that the combined oxide particlesare not mere mixed particles of aluminum oxide particles and siliconoxide particles, but aluminum oxide and silicon oxide are combined witheach other in the particle. Accordingly, it is conceivable that thecharge easily transfers in the particle to effectively prevent excessivecharge of the toner particles.

As for the use ratio of the surface-modified silica particles (A) andthe combined oxide particles (B), it is preferable that (A):(B) is from2/50 to 50/2 by weight ratio, and that the total amount of both thesurface-modified silica particles and the combined oxide particles addedis preferably from 0.01 to 1% by weight, more preferably from 0.02 to0.8% by weight, per the toner mother particles. Even when thesurface-modified silica particles are independently added to the tonerparticles coated with the negatively chargeable silica particles, thecharge is insufficiently maintained. Further, when the combined oxideparticles are independently added to the toner particles coated with thenegatively chargeable silica particles, the prevention of excess chargeis insufficiently maintained.

In the present invention, the surface-modified silica particles and thecombined oxide particles are externally added to the toner particlescoated with the negatively chargeable silica particles, thereby beingable to decrease the positively charged toner amount of the tonerparticles to prepare toner particles suitable for full colorization. Inthe surface-modified silica particles or the combined oxide particlesalone, the titanium, tin, zirconium or aluminum component in thesurface-modified silica particles, or the aluminum oxide component inthe combined oxide particles functions as a positive charge site, whichcauses the problem that the reverse transfer toner is developed toincrease fogging, furthermore leading to a decrease in transferefficiency.

Further, when the surface-modified silica particles or the combinedoxide particles are used in combination with the silica particles, theamount of external additives can be reduced compared to the conventionaladdition system of silica, titania, alumina or the like, so that thefixability is not lowered.

In the present invention, it is possible to use various other inorganicand organic external additives for toner in combination with theabove-mentioned external additives, as long as the functions thereof arenot impaired. Examples thereof include positively chargeable silica,alumina, zinc oxide, magnesium fluoride, silicon carbide, boron carbide,titanium carbide, zirconium carbide, boron nitride, titanium nitride,zirconium nitride, zirconium oxide, calcium carbonate, magnetite,molybdenum disulfide, a metal titanate such as strontium titanate, asilicon metal salt, and fine particles of a resin such as an acrylicresin, a styrene resin or a fluororesin.

In the present invention, it is possible to use various other inorganicand organic external additives for toner in combination with theabove-mentioned external additives, as long as the functions thereof arenot impaired. Examples thereof include positively chargeable silica,alumina, zinc oxide, magnesium fluoride, silicon carbide, boron carbide,titanium carbide, zirconium carbide, boron nitride, titanium nitride,zirconium nitride, zirconium oxide, calcium carbonate, magnetite,molybdenum disulfide, a metal titanate such as strontium titanate, asilicon metal salt, and fine particles of a resin such as an acrylicresin, a styrene resin or a fluororesin.

Further, metal soap particles are preferably added as the externaladditive particles, which lowers the number liberating rate of theexternal additive particles, prevents the occurrence of fogging, andmakes it possible to prevent scratches from occurring on a surface ofthe photoreceptor and to improve the transfer efficiency.

The metal soap particles are a higher fatty acid salt of a metalselected from zinc, magnesium, calcium and aluminum, and examplesthereof include magnesium stearate, calcium stearate, zinc stearate,monoaluminum stearate and trialuminum stearate. The mean particle sizeof the metal soap particles is preferably from 0.5 to 20 μm, and morepreferably from 0.8 to 10 μm.

The amount of the metal soap particles added is preferably 0.05 to 0.5%by weight, and more preferably from 0.1 to 0.3% by weight, based on thetoner mother particles. Less than 0.05% by weight results ininsufficient functions as a lubricant and a binder, whereas exceeding0.5% by weight results in the tendency of fogging to increase inreverse. It is preferable that the metal soap particles are added in anamount of 2 to 10% by weight based on the above-mentioned externaladditives. Less than 2% by weight unfavorably shows no effects as alubricant and a binder, whereas exceeding 10% by weight unfavorablyleads to a reduction in fluidity and an increase in fogging.

Then, as a method for externally adding various external additives tothe toner mother particles, it is preferable to first externally add twokinds of hydrophobic silica particles different in particle size to thetoner mother particles, and then, externally add the metal soapparticles together with the surface-modified silica particles and thecombined oxide particles. The work function of the hydrophobic silicaparticles is preferable from 5.0 to 5.3 eV, when measured by a methoddescribed later, and the work function of the toner mother particles ispreferably from 5.3 to 5.8 eV. In the negatively chargeable toner, theexternal additive particles having a smaller work function are fixedlyadhered to surfaces of the toner mother particles by charge transfercaused by the difference in the work function. Then, it is conceivablethat the surface-modified silica particles and the combined oxideparticles added in the after process are fixedly adhered to the silicaparticles on surfaces of the toner particles through the silicacomponent thereof and to surfaces of the toner mother particles.

Further, the metal soap particles are adhered to the vicinities of thesilica particles, surface-modified silica particles and combined oxideparticles on the surfaces of the toner mother particles, or directly tothe surfaces of the toner mother particles. However, by adjusting thework function of the toner mother particles approximately to that of themetal soap particles (the difference in absolute value is within 0.15eV), it conceivably becomes possible (1) to maintain the fluidity andcharge property of the toner mother particles without inhibiting thecharacteristics of giving the fluidity and charge property, which arefunctions of the inorganic additive particles, (2) to more decrease thenumber liberating rate of the external additive particles to moreprevent the occurrence of fogging, because the charge transfer in theexternal additive particles is not inhibited, and (3) to easily transferthe metal soap particles from the toner particles to a surface of alatent image carrier, to more prevent the occurrence of scratches on thesurface of the latent image carrier in cleaning and to improve thetransfer efficiency, because the adhesion of the metal soap particles tothe toner mother particles can be weakened.

Further, the work function of the surface-modified silica particlesadded in the after process is preferably from 5.2 to 5.5 eV, and thework function of the combined oxide particles indicates two kinds ofwork functions, a first work function ranging from 5.0 to 5.4 and asecond work functions ranging from 5.4 to 5.7, as described in reference5 previously filed by the present assignee. It is conceivable that boththe surface-modified silica particles and the combined oxide particlesare adhered to the vicinities of the silica particles on the surfaces ofthe toner particles, or to the toner mother particles.

The work function (Φ) is known as energy necessary for taking electronsout of a material. The smaller the work function is, the more easily theelectron is released, and the larger the work function is, the moredifficult the electron is to be released. Accordingly, when a materialhaving a smaller work function is brought into contact with a materialhaving a larger work function, the material having a smaller workfunction is positively charged, and the material having a larger workfunction is negatively charged. The work function is numericallyindicated as energy (eV) for taking electrons out of a material, and canevaluate the charge property by contact between various materials. Thework function (Φ) is measured using a surface analyzer (manufactured byRiken Keiki Co., Ltd., AC-2, a low-energy counting system). In thisanalyzer, a sample is irradiated, using a heavy hydrogen lump, settingthe dose of irradiating light to 500 nW, selecting a monochromic lightwith a spectrograph, and setting the irradiation area to 4 mm square,within the energy scanning range of 3.4 to 6.2 eV for a measuring timeof 10 sec/position. The work function (Φ) is determined by detectingphotoelectrons emitted from a surface of the sample, and measured with arepetition accuracy (standard deviation) of 0.02 eV. In order to ensurethe repeatability of data, the sample is used as a sample to be measuredafter it has been allowed to stand under conditions of a temperature of25° C. and an RH of 55% for 24 hours. Samples to be measured of thetoner mother particles, the external additive particles, the metal soapparticles and the toner particles are measured using a measuring cellfor toner exclusive use.

The external additive particles used in the present invention arepreferably subjected to hydrophobic treatment with a silane couplingagent, a titanium coupling agent, a higher fatty acid, a silicone oil orthe like to use. The hydrophobilization rate is preferaby 40% or more,and more preferably 50% or more. The hydrophobilizing agents include,for example, dimethyldichlorosilane, octyltrimethoxysilane,hexamethyldisilazane, silicone oil, octyltrichlorosilane,decyltrichlorosilane, nonyltrichlorosilane,(4-iso-propylphenyl)trichlorosilane, (4-t-butylphenyl)trichlorosilane,dipentyldichlorosilane, dihexyldichlorosilane, dioctyldichlorosilane,dinonyldichlorosilane, didecyldichlorosilane, didodecyldichlorosilane,(4-t-butylphenyl)octyldichlorosilane, didecenyldichlorosilane,dinonenyldichlorosilane, di-2-ethylhexyldichlorosilane,di-3,3-dimethylpentyldichlorosilane, trihexylchlorosilane,trioctylchlorosilane, tridecylchlorosilane, dioctylmethylchlorosilane,octyldimethylchlorosilane and (4-iso-propylphenyl)diethylchlorosilane.After coupling treatment, the particles may be further treated with asilicone oil or the like to further enhance hydorophobicity.

The amount of the external additive particles added as a whole ispreferably from 0.51 to 2.5% by weight, and more preferably from 0.8 to2.3% by weight, based on the toner mother particles. Less than 0.51% byweight results in no effect of imparting the fluidity and preventingexcessive charge, whereas exceeding 2.5% by weight results in a decreasein the amount of negative charge and concurrently in an increase in theamount of positively charged toner having the reverse polarity, whichincreases fogging and the amount of reverse transfer toner to results inunsuitability for full color applications.

In the method for producing the toner of the present invention, it ispreferred that the hydrophobic silica particles are first externallyadded to the toner mother particles, and then, the surface-modifiedsilica particles, the combined oxide particles or the metal soapparticles are externally added thereto, as described above. Eachexternal additive is preferably added to the toner mother particles witha Henschel mixer (manufactured by Mitsui Miike Machinery Co., Ltd.), amechanofusion system (manufactured by Hosokawa Micron Co., Ltd.) orMechanomill (manufactured by Okada Seiko Co., Ltd.). When the Henschelmixer is used, it is preferably operated at 5,000 to 7,000 rpm for 1 to3-minutes in addition of the hydrophobic silica particles in the firststep, and it is preferably operated at 5,000 to 7,000 rpm for 1 to 3minutes in addition of the surface-modified silica particles, thecombined oxide particles and the metal soap particles in the secondstep.

The work function of the negatively chargeable toner thus obtained ispreferably from 5.3 to 5.9 eV, and more preferably from 5.4 to 5.85 eV.Fogging can be more reduced, and the transfer efficiency can be moreimproved by increasing the work function of the negatively chargeabletoner more than the work function of the surface of the photoreceptor,as described in Examples. Further, in regulating the formation of a thinfilm of the toner on a developing roller with a toner regulating member,the “excessive charge” phenomenon that the amount of charge in thenegatively chargeable toner is extremely increased occurs in some times.However, when the work function of the negatively chargeable toner isdecreased, compared to the work function of the surface of thephotoreceptor, this phenomenon can be inhibited.

The negatively chargeable toner of the present invention, for the tonerobtained by the pulverization method, has a number-based mean particlesize of preferably from 5 to 10 μm, more preferably from 6 to 9 μm. Thenegatively chargeable toner of the present invention, for the tonerobtained by the polymerization method, preferably has a particle sizedistribution such that 50% or more of particles having a number-basedmean particle size of 8 μm or less, more preferably from 4.5 to 8 μm,and 10% or less, more preferably 5% or less of particles having anumber-base particle size of 3 μm or less.

In both cases of the pulverization method and the polymerization method,the negatively chargeable toner of the present invention preferably hasa sphericity (spheroidization coefficient) of 0.94 or more, morepreferably 0.95 or more. When the sphericity (spheroidizationcoefficient) is up to 0.97, a cleaning blade is preferably used, andwhen it is 0.97 or more, brush cleaning is preferably used incombination therewith. The transfer efficiency can be improved byadjusting the sphericity (spheroidization coefficient) of the toner to0.94 or more.

The mean particle size and the sphericity (spheroidization coefficient)of the toner mother particles and the toner particles are valuesmeasured with an “FPIA2100” analyzer manufactured by Sysmex Corporation.Further, the mean particle size of the external additive particles is avalue measured by electronography.

The image forming apparatus of the present invention will be describedbelow. FIG. 1 shows an embodiment of a contact developing process in theimage forming apparatus using the toner of the present invention.

A photoreceptor 1 is a photoreceptor drum which generally has a diameterof 24 to 86 mm and generally rotates at a surface speed of 60 to 300mm/sec, and after a surface thereof has been uniformly negativelycharged with a corona charging device 2, exposure to light 3 is carriedout depending on information to be recorded, thereby forming anelectrostatic latent image. A developing device 10 is a single-componentdeveloping apparatus, which supplies a single-component non-magnetictoner T onto the photoreceptor 1, thereby reversely developing theelectrostatic latent image on the photoreceptor 1 to form a visibleimage. The single-component non-magnetic toner T is contained in adeveloping means, and supplied to a developing roller 9 with a tonersupply roller 7 which rotates counterclockwise as shown in FIG. 1. Thedeveloping roller 9 rotates counterclockwise, and conveys the toner Tsupplied with the toner supply roller 7 to a contact portion with thephotoreceptor 1, with the toner T held on a surface thereof. Then, theelectrostatic latent image on the photoreceptor 1 is made visible.

The developing roller 9 is, for example, a roller having a diameter of16 to 24 mm in which a metal pipe is plated or treated by blasting, orin which a conductive elastic layer having a volume resistance of 10⁴ to10⁸ Ω·cm and a hardness (Asker A hardness) of 40 to 70° is generallyformed on a central axis peripheral surface of a metal pipe, theconductive elastic layer comprising a butadiene rubber, astyrene-butadiene rubber, an ethylene-propylene rubber, a urethanerubber, a silicone rubber or the like. Developing bias voltage isapplied from a power source (not shown) through a shaft of this pipe.Further, the developing device 10 comprising the developing roller 9,the toner supply roller 7 and a toner regulating blade 8 is preferablypressed to the photoreceptor 1 with a biasing means such as a spring(not shown) with a pressing force of 19.6 to 98.1 N/m, more preferably24.5 to 68.6 N/m so as to give a nip width of 1 to 3 mm.

As the regulating blade 8, there is used a blade obtained by laminatingrubber tips with stainless steel, phosphor bronze, a rubber plate or athin metal plate. The blade is preferably pressed to the developingroller 9 with a biasing means such as a spring (not shown) or utilizingrepulsive force as an elastic material, with a preferable line pressureof 245 to 490 mN/cm, thereby forming two or more toner layers on thedeveloping roller.

In the contact developing process, the dark potential of photoreceptor 1is preferably from −500 to −700 V, and the light potential thereof ispreferably from −50 to −150 V. Although not shown, the developing biasvoltage is preferably −100 to −400 V, and the developing roller 9 andthe toner supply roller 7 preferably have the same potential.

In the contact developing process, the ratio of the peripheral speed ofthe developing roller 9 which rotates counterclockwise to that of thephotoreceptor 1 which rotates clockwise is preferably set to 1.2 to 2.5,more preferably 1.5 to 2.2, thereby being able to make sure contactfrictional charge with the photoreceptor 1 even when the toner particlesare small in size.

There is no particular limitation on the relationship between therespective work functions of the regulating blade 8 and the developingroller 9 and the work function of the toner. However, the respectivework functions of the regulating blade 8 and the developing roller 9 aremade smaller than the work function of the toner to cause negativecontact charge in the toner in contact with the regulating blade 8,thereby being able to obtain the more uniformly negatively chargedtoner. Further, voltage may be applied to the regulating blade 8 toinject charge into the toner, thereby controlling the amount of chargein the toner.

The intermediate transfer medium used in the image forming apparatus ofthe present invention will be described below. In FIG. 1, anintermediate transfer medium 4 is sent between the photoreceptor 1 and aback-up roller 6, and a visible image on the photoreceptor 1 istransferred onto the intermediate transfer medium 4 by application ofvoltage to form a toner image on the intermediate transfer medium 4. Thetoner remaining on the photoreceptor 1 is removed with a cleaning blade5, and electrostatic charge on the photoreceptor 1 is erased with anerasing lamp. Thus, the photoreceptor 1 is reused. In the image formingapparatus of the present invention, reversely charged toner particlescan be inhibited, so that the amount of the toner remaining on thephotoreceptor 1 can be decreased, thereby being able to minimize acleaning toner container. The pressing force of the intermediatetransfer medium 4 to the photoreceptor 1 by the back-up roller 6 ispreferably from 18.8 to 45.2 N/m, more preferably from 26.3 to 37.7 N/m.

When a transfer drum or a transfer belt is used as the intermediatetransfer medium 4, a voltage of +250 to +600 V is preferably applied asprimary transfer voltage to a conductive layer thereof, and in secondarytransfer to a transfer material such as paper, a voltage of +400 to+2,800 V is preferably applied as secondary transfer voltage.

The transfer drum or the transfer belt can be used as the intermediatetransfer medium. As the transfer belt, one is a belt in which a transferlayer is provided on a film or a sheet comprising a substrate made froma synthetic resin, and the other is a belt in which a transfer layer isprovided as a surface layer on a base layer of an elastic material. Asthe transfer drum, when an organic photosensitive layer is provided on adrum having rigidity, for example, a drum made of aluminum, a transferlayer which is an elastic surface layer is provided on a drum substratehaving rigidity such as an aluminum substrate to form the transfermedium. Further, when a support of the photoreceptor is in belt form, ora so-called elastic photoreceptor in which a photosensitive layer isprovided on an elastic support such as a rubber support, a transferlayer is preferably provided on a drum having rigidity, for example, adrum made of aluminum, directly or with the interposition of aconductive intermediate layer. As the substrate, there can be used aconductive or insulating substrate. In the case of the transfer belt,the volume resistance is preferably within the range of 10⁴ to 10¹²Ω·cm, more preferably 10⁶ to 10¹¹ Ω·cm.

As for a material suitable for the film or the sheet and a method forpreparing the same, a conductive material such as conductive carbonblack, conductive titanium oxide, conductive tin oxide or conductivesilica is dispersed in an engineering plastic resin such as a modifiedpolyimide, a thermosetting polyimide, a polycarbonate, anethylene-tetrafluoroethylene copolymer, polyvinylidene fluoride or anylon alloy, the resulting resin composition is extruded to form asemiconductive film generally having a thickness of 50 to 500 μm, ormolded to form a seamless substrate, and a fluororesin coating generallyhaving a thickness of 5 to 50 μm is formed on an outer side thereof as asurface protective layer for reducing surface energy and preventingfilming of the toner, thereby obtaining a seamless belt.

As a method for forming the surface protective layer, there can be useda dip coating method, a ring coating method, a spray coating method orthe like. In order to prevent cracking and elongation at edges of thetransfer belt and a meandering movement thereof, tapes made of apolyethylene terephthalate film generally having a thickness of 80 μm orribs made of a urethane rubber are attached on both edges of thetransfer belt.

When the substrate is prepared from the film or the sheet, in order toform a belt-like substrate, edges thereof are ultrasonic welded, therebybeing able to prepare a belt. Specifically, a conductive layer and asurface layer are provided on the sheet or the film, and then,ultrasonic welding is conducted, thereby being able to prepare thetransfer belt having desired physical properties. More specifically,when a polyethylene terephthalate film having a thickness of 60 to 150μm is used as the insulating substrate, aluminum is deposited over asurface thereof, or an intermediate layer comprising a resin and aconductive material such as carbon black is further formed thereon bycoating, and a semiconductive surface layer comprising a urethane resin,a fluororesin and a conductive material, which has a surface resistancehigher than that of the intermediate surface layer, is provided thereon,thereby being able to form the transfer belt. When a resistive layer canbe provided which does not require such a large amount of heat in dryingafter coating, it is also possible to provide the above-mentionedresistive layer after the ultrasonic welding of the aluminum-depositedfilm, thereby preparing the transfer belt.

As for a material suitable for the elastic substrate such as a rubberand a method for preparing the same, the above-mentioned conductivematerial is dispersed in a silicone rubber, a urethane rubber, a nitrilerubber, an ethylene-propylene rubber or the like, the resulting rubbercomposition is extruded to form a semiconductive rubber belt generallyhaving a thickness of 0.8 to 2.0 mm, and then, a surface thereof ispolished with an abrasive such as a sand paper or a polisher to controlthe surface roughness to a desired value. Although an elastic layerobtained at this time may be used as such, a surface protective layercan be provided in a similar manner as described above.

In the case of the transfer drum, the volume resistance is preferablywithin the range of 10⁴ to 10¹² Ω·cm, more preferably 10⁷ to 10¹¹ Ω·cm.The transfer drum can be prepared by providing a conductive intermediatelayer of an elastic material on a cylinder of a metal such as aluminumas needed to form a conductive elastic substrate, and forming thereon,for example, a fluororesin coating generally having a thickness of 5 to50 μm as a surface protective layer for reducing surface energy andpreventing filming of the toner.

As the conductive elastic substrate, for example, a conductive materialsuch as carbon black, conductive titanium oxide, conductive tin oxide orconductive silica is blended with, kneaded with and dispersed in arubber material such as a silicone rubber, a urethane rubber, a nitrilerubber (NBR), an ethylene-propylene rubber (EPDM), a butadiene rubber, astyrene-butadiene rubber, an isoprene rubber, a chloroprene rubber, abutyl rubber, an epichlorohydrin rubber or a fluororubber, and theresulting conductive rubber material is molded so as to adhere to analuminum cylinder having a preferable diameter of 90 to 180 mm, therebyforming a layer having a preferable thickness after polishing of 0.8 to6 mm and a preferable volume resistance of 10⁴ to 10¹⁰ Ω·cm. Then, asemiconductive surface layer preferably having a thickness of about 15to 40 μm, which comprises a urethane resin, a fluororesin, a conductivematerial and fluorine-based resin particles, thereby being able to formthe transfer drum having a desired volume resistance of 10⁷ to 10¹¹Ω·cm. The surface roughness thereof at this time is preferably 1 μm (Ra)or less. Further, as another example, it is also possible to cover theconductive elastic substrate prepared as described above with asemiconductive tube of a fluororesin or the like and to allow the tubeto contract by heating, thereby preparing the transfer drum having thedesired surface layer and electric resistance.

Then, FIG. 2 shows an example of a non-contact developing process in theimage forming apparatus using the toner of the present invention. Inthis process, a developing roller 9 and a photoreceptor 1 face eachother through a developing gap d. The developing gap is preferably from100 to 350 μm. Further, although not shown, the developing bias of DCvoltage is preferably from −200 to −500 V, AC voltage superimposedthereon is preferably from 1.5 to 3.5 kHz, and P-P voltage is preferablyfrom 1,000 to 1,800 V. Furthermore, in the non-contact developingprocess, the ratio of the peripheral speed of the developing rollerwhich rotates counterclockwise to that of an photoreceptor 1 whichrotates clockwise is preferably set to 1.0 to 2.5, more preferably 1.2to 2.2.

The developing roller 9 rotates counterclockwise as shown in FIG. 2, andconveys a toner T supplied with a toner supply roller 7 to a portionopposite to the photoreceptor 1, with the toner T adsorbed on a surfacethereof. The toner T vibrates between a surface of the developing roller9 and a surface of the photoreceptor 1 by applying the AC voltage by asuperposition in a portion where the photoreceptor 1 and the developingroller 9 face each other, thereby conducting development. In the presentinvention, the toner particles can be brought into contact with thephotoreceptor 1 while the toner T vibrates between the surface of thedeveloping roller 9 and the surface of the photoreceptor 1 byapplication of the AC voltage. Accordingly, it is conceivable thatsmall-sized positively charged toner particles can be negativelycharged, thereby being able to reduce fogging.

An intermediate transfer medium 4 is sent between the photoreceptor 1 onwhich an image is visualized and a back-up roller 6. The pressing forceof the intermediate transfer medium to the photoreceptor 1 by theback-up roller 6 is preferably increased about 30% compared to thecontact developing process, that is to say, preferably from 24.5 to 58.8N/m, more preferably from 34.3 to 49 N/m.

Further, the peripheral velocity ratio of the photoreceptor 1 to theintermediate transfer medium 4 is preferably from 0.95 to 1.05.

This can make sure the contact between the toner particles and thephotoreceptor 1, and the toner particles can be more negatively chargedto improve the transfer efficiency.

Items other than the above in the non-contact developing process are thesame as with the contact developing process described above.

When developing devices using four color toners (developing agents)comprising yellow Y, cyan C, magenta M and black K are combined with thephotoreceptor in the developing processes shown in FIGS. 1 and 2, anapparatus which can form a full color image can be provided.

The full color image forming apparatus to which the negativelychargeable toner of the present invention is applied will be describedbelow. FIG. 3 is a view for illustrating an embodiment of a full colorprinter of a 4-cycle system.

In FIG. 3, the reference numeral 100 designates an image carriercartridge into which an image carrier unit is incorporated. In thisembodiment, the image carrier cartridge is constituted as aphotoreceptor cartridge, and a photoreceptor and developing units can beseparately mounted. An electrophotographic photoreceptor (latent imagecarrier) 140 is driven for rotation in the direction indicated by thearrow in FIG. 3 with an appropriate driving means not shown. Around thephotoreceptor 140, a charging roller 160 as a charging means, developingdevices 10 (Y, M, C, K) as developing means, an intermediate transferdevice 30 and a cleaning unit 170 are arranged along its rotationaldirection.

The charging roller 160 is brought into abutting contact with an outerperipheral surface of the photoreceptor 140 to uniformly charge theouter peripheral surface. The uniformly charged outer peripheral surfaceof the photoreceptor 140 is selectively exposed to light L1corresponding to desired image information with an exposure unit 40, andthis exposure to light L1 forms an electrostatic latent image on thephotoreceptor 140. This electrostatic latent image is developed byadding developing agents with the developing devices 10.

As the developing devices, there are provided a developing device 10Yfor yellow, a developing device 10M for magenta, a developing device 10Cfor cyan and a developing device 10K for black. These developing devices10Y, 10M, 10C and 10K are each rockably constituted so that only adeveloping roller 9 of one developing device is selectively brought intopress contact with the photoreceptor 140. These developing devices 10each hold negatively charged toners on the respective developingrollers, and give any one toner of yellow Y, magenta M, cyan C and blackK to a surface of the photoreceptor 140 to develop the electrostaticlatent image on the photoreceptor 140. The developing roller 9 isconstituted by a hard roller, for example, a metal roller whose surfaceis roughened. The developed toner image is transferred onto anintermediate transfer belt 36 of the intermediate transfer device 30.The cleaning unit 170 comprises a cleaner blade for scrapping off thetoner T adhering to the outer peripheral surface of the photoreceptor140 after the above-mentioned transfer, and a cleaning toner-collectingmember for receiving the toner scrapped off with the cleaner blade.

The intermediate transfer device 30 comprises a driving roller 31, fourdriven rollers 32, 33, 34 and 35, and an endless intermediate transferbelt 36 laid around these rollers under tension. A gear (not shown)fixed at an end of the driving roller 31 engages with a driving gear ofthe photoreceptor 140, whereby the driving roller 31 is driven forrotation at a peripheral speed approximately similar to that of thephotoreceptor 140. Accordingly, the intermediate transfer belt 36 isdriven for circulation at a peripheral speed approximately similar tothat of the photoreceptor 140 in the direction indicated by the arrow inFIG. 3.

The driven roller 35 is disposed at such a position that theintermediate transfer belt 36 is brought into press contact with thephotoreceptor 140 by the tension of itself between the driven roller 35and the driving roller 31, and a primary transfer portion T1 is formedat a position at which the intermediate transfer belt 36 is brought intopress contact with the photoreceptor 140. The driven roller 35 isdisposed near the primary transfer portion T1 upstream in thecirculating direction of the intermediate transfer belt.

An electrode roller (not shown) is disposed on the driving roller 31with the interposition of the intermediate transfer belt 36, and primarytransfer voltage is applied to a conductive layer of the intermediatetransfer belt 36 through this electrode roller. The driven roller 32 isa tension roller, and urges the intermediate transfer belt 36 in atensioning direction thereof with a biasing means not shown. The drivenroller 33 is a back-up roller for forming a secondary transfer portionT2. A secondary transfer roller 38 is disposed opposite to this back-uproller 33 with the interposition of the intermediate transfer belt 36.Secondary transfer voltage is applied to the secondary transfer roller,which is constituted so that the clearance between the secondarytransfer roller and the intermediate transfer belt 36 is adjustable witha clearance adjusting mechanism not shown. The driven roller 34 is aback-up roller for a belt cleaner 39. The belt cleaner 39 is constitutedso that the clearance between the belt cleaner and the intermediatetransfer belt 36 is adjustable with a clearance adjusting mechanism notshown.

The intermediate transfer belt 36 comprises a multilayer belt having aconductive layer and a resistive layer formed thereon and brought intopress contact with the photoreceptor 140. The conductive layer is formedon an insulating substrate formed of a synthetic resin, and the primarytransfer voltage is applied to this conductive layer through theabove-mentioned electrode roller. The resistive layer is removed instrip form along a side edge of the belt, thereby exposing theconductive layer in strip form. The electrode roller is arranged so asto come in contact with the exposed portion.

In the course in which the intermediate transfer belt 36 is driven forcirculation, the toner image on the photoreceptor 140 is transferredonto the intermediate transfer belt 36 at the primary transfer portionT1, and the toner image transferred onto the intermediate transfer belt36 is transferred to a recording medium S such as paper which issupplied between the intermediate transfer belt and the secondarytransfer roller 38 at the secondary transfer portion T2. The sheet S isfed from a sheet paper feeder 50, and supplied to the secondary transferportion T2 with a pair of gate rollers G at a predetermined timing. Thereference numeral 51 designates a paper feed cassette, and the referencenumeral 52 designates a pickup roller.

The toner image is fixed at the secondary transfer portion T2, and thesheet is discharged through a delivery path 70 onto a sheet receivingportion 81 formed on a casing 80 of a main body of the apparatus. Thisimage forming apparatus has two delivery paths 71 and 72 which areindependent from each other, as the delivery path 70, and the sheetwhich has passed through a fixing device 60 is discharged through eitherthe delivery path 71 or 72. Further, the delivery paths 71 and 72 alsoform a switchback path. When images are formed on both sides of thesheet, the sheet which has once entered the delivery path 71 or 72 issupplied again to the secondary transfer portion T2 through a returnroller 73.

The outline of the operations of the whole image forming apparatus asdescribed above is as follows:

-   -   (1) When image information is transmitted from a personal        computer or the like (not shown) to a control unit 90 of the        image forming apparatus, the photoreceptor 140, the respective        rollers 9 of the developing devices 10 and the intermediate        transfer belt 36 are driven for rotation.    -   (2) The outer peripheral surface of the photoreceptor 140 is        uniformly charged by the charging roller 160.    -   (3) The uniformly charged outer peripheral surface of the        photoreceptor 140 is selectively exposed to light L1        corresponding to image information of a first color (for        example, yellow) with the exposure unit 40, thereby forming an        electrostatic latent image for yellow.    -   (4) Only the developing roller of the developing device 10Y for        the first color, for example, yellow, is brought into contact        with the photoreceptor 140, thereby developing the        above-mentioned electrostatic latent image to form a toner image        of the first color, yellow, on the photoreceptor 140.    -   (5) The primary transfer voltage having a charge polarity        opposite to that of the above-mentioned toner is applied to the        intermediate transfer belt 36, and the toner image formed on the        photoreceptor 140 is transferred onto the intermediate transfer        belt 36 at the primary transfer portion T1. At this time, the        secondary transfer roller 38 and the belt cleaner 39 are kept        away from the intermediate transfer belt 36.    -   (6) After the toner remaining on the photoreceptor 140 has been        removed with a cleaning means 170, the charge of the        photoreceptor 140 is removed by charge removing light L2 from a        removing means 41.    -   (7) The above-mentioned operations (1) to (6) are repeated as        needed. That is to say, according to print command signals, the        operations are repeated for a second, third and fourth colors,        and toner images corresponding to the above-mentioned print        command signals are superposed on each other on the intermediate        transfer belt 36 to form a full color image.    -   (9) The sheet S is fed from a sheet paper feeder 50 at a        predetermined timing, and the toner image on the intermediate        transfer belt 36, that is to say, the full color image formed by        superimposing the four color toner images, is transferred onto        the sheet S with the secondary transfer roller 38, immediately        before or after an end of the sheet S has reached the secondary        transfer portion T2, namely, at the timing when the toner image        on the intermediate transfer belt 36 is transferred to a desired        position on the sheet S. Further, the belt cleaner 39 is brought        into abutting contact with the intermediate transfer belt 36 to        remove the toner remaining on the intermediate transfer belt 36        after the secondary transfer.    -   (9) The sheet S passes through the fixing device 60, thereby        fixing the toner image on the sheet S. Then, the sheet S is        conveyed toward a predetermined position (toward the sheet        receiving portion 81 in the case of no double-sided printing, or        toward the return roller 73 through the switchback path 71 or 72        in the case of double-sided printing).

In the image forming apparatus according to the present invention, thedeveloping roller 9 and the intermediate transfer medium 36 may be inabutting contact with the photoreceptor 140, and development may beconducted by the non-contact process.

Similarly, a schematic front view of a full color printer of a tandemsystem used in the present is shown in FIG. 4. In this case, thephotoreceptor and the developing unit are constituted so as to bemounted as one unit, that is to say, a process cartridge which can bedetachably mounted on a main body of the image forming apparatus. As fordevelopment, an example of the contact process is shown, but thenon-contact process can also be employed.

This image forming apparatus comprises an intermediate transfer belt 30laid around only two rollers, namely, a driving roller 11 and a drivenroller 12, under tension, and driven for circulation in the directionindicated by the arrow in FIG. 4 (counterclockwise), and fourmonochromatic toner image forming units 20(Y), 20(C), 20(M) and 20(K)arranged along the intermediate transfer belt 30. The image formingapparatus is constituted so that toner images formed with the pluralityof monochromatic toner image forming units 20 are sequentially primarilytransferred to the intermediate transfer belt 30 with individualtransfer means 13, 14, 15 and 16. The respective primary transferportions are indicated by T1Y, T1C, T1M and T1K.

AS the monochromatic toner image forming units, there are arranged theunit 20(Y) for yellow, the unit 20(M) for magenta, the unit 20(C) forcyan and the unit 20(K) for black. These monochromatic toner imageforming units 20(Y), 20(C), 20(M) and 20(K) each comprises aphotoreceptor 21 having a photosensitive layer on its outer peripheralsurface, a charging roller 22 as a charging mean for uniformly chargingthe outer peripheral surface of the photoreceptor 21, an exposure unit23 for selectively exposing the outer peripheral surface uniformlycharged with the charging roller 22 to form an electrostatic latentimage, a developing roller 24 as a developing means for imparting adeveloping agent or a toner to the electrostatic latent image formedwith the exposure unit 23 to form a visible image (toner image), and acleaning blade 25 as a cleaning means for removing the toner remainingon the surface of the photoreceptor 21 after the toner image developedwith the developing roller 24 has been transferred to the intermediatetransfer belt 30 for primary transfer.

These monochromatic toner image forming units 20(Y), 20(C), 20(M) and20(K) are arranged on a loose side of the intermediate transfer belt 30.The toner images are sequentially primarily transferred to theintermediate transfer belt 30, and sequentially superposed on each otheron the intermediate transfer belt 30 to form a full color toner image.The full color toner image is secondarily transferred to a recordingmedium S such as a paper sheet at a secondary transfer portion T2, andfixed onto the recording medium S by passing between a pair of fixingrollers 61. Then, the recording medium S is discharged with a pair ofdelivery rollers 62 to a predetermined place, that is to say, a deliverytray or the like (not shown). The reference numeral 51 designates apaper feed cassette in which the recording media S are held in thestacked state, 52 designates a pickup roller for feeding the recordingmedia S sheet by sheet from the paper feed cassette 51, and G designatesa pair of gate rollers for defining a paper feed timing of the recordingmedium S to the secondary transfer portion T2.

Further, the reference numeral 63 designates a secondary transfer rolleras a secondary transfer means for forming the secondary transfer portionT2 between the secondary transfer roller and the intermediate transferbelt 30, and 64 designates a cleaning blade as a cleaning means forremoving the toner remaining on a surface of the intermediate transferbelt 30 after secondary transfer. The cleaning blade 64 after secondarytransfer is in abutting contact with the intermediate transfer belt 30at a portion at which the intermediate transfer belt 30 is wrappedaround the driving roller 11, not around the driven roller 12.

EXAMPLES

The present invention is now illustrated in greater detail withreference to Examples and Comparative Examples, but it should beunderstood that the present invention is not to be construed as beinglimited thereto.

Methods for preparing toner mother particles used in Examples describedlater and the like will be shown below.

Preparation of Toner Mother Particles 1

A monomer mixture of 80 parts by weight of a styrene monomer, 20 partsby weight of butyl acrylate and 5 parts by weight of acrylic acid wasadded to an aqueous mixture of 105 parts by weight of water, 1 part byweight of a nonionic emulsifier, 1.5 parts by weight of an anionicemulsifier and 0.55 part by weight of potassium persulfate.

Then, polymerization was conducted with stirring at 70° C. in a streamof nitrogen for 8 hours. The reaction product was cooled after thepolymerization reaction to obtain a milky white resin emulsion having aparticle size of 0.25 μn.

Then, a mixture of 200 parts by weight of the resulting resin emulsion,20 parts by weight of a polyethylene wax emulsion (manufactured by SanyoChemical Industries, Ltd., Permarin PN) and 7 parts by weight ofPhthalocyanine Blue was dispersed in water containing 0.2 part by weightof sodium dodecylbenzenesulfonate, and diethylamine was added to adjustthe pH to 5.5. Thereafter, 0.3 part by weight of aluminum sulfate wasadded thereto with stirring, and then, dispersed by high-speed stirringusing an emulsifying and dispersing apparatus (manufactured by TokushuKika Kogyo Co., Ltd., TK Homomixer).

Further, 40 parts by weight of styrene monomer, 10 parts by weight ofbutyl acrylate, 5 parts by weight of zinc salicylate were added togetherwith 40 parts by weight of water, and similarly heated at 90° C. withstirring in a stream of nitrogen. Then, hydrogen peroxide was added andpolymerization was conducted for 5 hours to allow particles to grow.After the polymerization was terminated, the temperature was elevated to95° C. while adjusting the pH to 5 or more, and maintained for 5 hours,for increasing the bond strength of associated particles. Thereafter,the resulting particles were washed with water, and dried under vacuumat 45° C. for 10 hours.

The resulting cyan toner was a toner having a number-based mean particlesize of 6.8 μm and a sphericity of 0.980. The work function of theresulting toner mother particles 1 was measured. As a result, it was5.57 eV.

Preparation of Toner Mother Particles 2

A magenta toner was obtained in the same manner as with the preparationof toner mother particles 1 with the exception that quinacridone wasused in place of Phthalocyanine Blue, and that in order to enhance theassociation of secondary particles and the film-forming bond strengththereof, the temperature was kept at 90° C. without elevation to 95° C.The resulting magenta toner was a toner having a number-based meanparticle size of 7.0 μm and a sphericity of 0.976. The work function ofthe resulting toner mother particles 2 was measured. As a result, it was5.64 eV.

Preparation of Toner Mother Particles 3 and 4

Polymerization was conducted in the same manner as with the preparationof toner mother particles 2 with the exception that Pigment Yellow 180and carbon black were each used in place of Phthalocyanine Blue in thepreparation of toner mother particles 1, thereby obtaining a yellowtoner and a black toner, respectively.

The resulting yellow toner was a toner having a number-based meanparticle size of 6.9 μm and a sphericity of 0.973. The work function ofthe resulting toner mother particles 3 was measured. As a result, it was5.59 eV.

Further, the resulting black toner was a toner having a number-basedmean particle size of 7.0 μm and a sphericity of 0.974. The workfunction of the resulting toner mother particles 4 was measuredsimilarly. As a result, it was 5.52 eV.

Preparation of Toner Mother Particles 5

A hundred parts by weight of a 50:50 (by weight) mixture (manufacturedby Sanyo Chemical Industries, Ltd., Himer ES-803) of a polycondensationpolyester of an aromatic dicarboxylic acid and alkylene etherifiedbisphenol A, and a partially crosslinked product of the polycondensationpolyester with a multivalent metal compound, 5 parts by weight ofPigment Blue 15:1, a cyan pigment, 3 parts by weight of polypropylene(melting point: 152° C., weight average molecular weight: 4,000) as arelease agent and 4 parts by weight of a metal complex of salicylic acid(manufactured by Orient Chemical Industries, Ltd., E-81) as a chargecontrol agent were homogeneously mixed using a Henschel mixer, thenkneaded by a twin-screw extruder having an internal temperature of 140°C., and cooled. The cooled product was pulverized to crude particles of2 mm square, which was further finely pulverized with a jet mill. Theresulting fine particles were classified with a classifying apparatus byrotor rotation to obtain a classified toner having a number-based meanparticle size of 6.2 μm and a sphericity of 0.905.

Hydrophobic silica (mean primary particle size: 7 nm, specific surfacearea: 250 m²/g) was added to the classified toner at a weight ratio of0.2% to conduct surface treatment. Then, using a hot air spheroidizingapparatus “Therfusion System”, the heat treatment temperature was set to250° C., and partial spheroidizing treatment was conducted. Thereafter,the resulting toner particles were classified again with a classifyingapparatus by rotor rotation to obtain a cyan toner having a number-basedmean particle size of 6.3 μm and a sphericity of 0.943. The workfunction of the resulting toner mother particles 5 was measuredsimilarly. As a result, it showed 5.46 eV.

Preparation of Toner Mother Particles 6, 7 and 8

Pulverization, classification, surface treatment by heat treatment andreclassification were conducted in the same manner as with thepreparation of toner mother particles 5 with the exception that thepigment was substituted by 6B of the Naphthol AS series to obtain amagenta toner having a number-based mean particle size of 6.6 μm and asphericity of 0.943. The work function of the resulting toner motherparticles 6 was measured similarly. As a result, it showed 5.53 eV.

Similarly, toner mother particles 7 (Pigment Yellow 93 was used as ayellow toner) and toner mother particles 8 (carbon black was used as ablack toner) were each prepared. The mean particle size and thesphericity of the resulting toner mother particles showed values similarto those of toner mother particles 6. Further, the work function oftoner mother particles 7 was 5.57, and the work function of toner motherparticles 8 was 5.63.

Preparation of Developing Roller

Nickel plating having a thickness of 10 μm was applied onto a surface ofan aluminum pipe having an outer diameter of 18 mm. The surfaceroughness (Ra) of the plated layer was 4 μm. The surface of theresulting developing roller was partially cut, and the work functionthereof was measured in the same manner as with an organicphotoreceptor. As a result, it was 4.58 eV.

Preparation of Regulating Blade A conductive urethane rubber piecehaving a thickness of 1.5 mm was adhered to a SUS plate having athickness of 80 μm with a conductive adhesive to produce a regulatingblade. The work function of the polyurethane rubber portion was measuredin the same manner as with an organic photoreceptor. As a result, it wasabout 5 eV.

Preparation of Transfer Belt 1

A homogeneous liquid dispersion of 30 parts by weight of a vinylchloride-vinyl acetate copolymer, 10 parts by weight of conductivecarbon black and 70 parts by weight of methanol was applied onto a 130μm-thick, aluminum-deposited polyethylene terephthalate film by rollcoating so that the coated film after drying has a thickness of 20 μm,and dried to prepare an intermediate conductive layer.

Then, a liquid dispersion obtained by mixing and dispersing 55 parts byweight of a nonionic aqueous urethane resin (solid content: 62%), 11.6parts by weight of a polytetrafluoroethylene emulsion (solid content:60%), 25 parts by weight of conductive tin oxide, 34 parts by weight offine polytetrafluoroethylene particles (maximum particle size: 0.3 μm orless), 5 parts by weight of a polyethylene emulsion (solid content: 35%)and 20 parts by weight of ion exchanged water was applied onto theintermediate conductive layer by roll coating so as to give a thicknessof 10 μm after drying, and dried. A film in which the coated film wasformed was cut to a length of 540 mm. Both ends were overlapped witheach other and welded by ultrasonic welding, thereby preparing atransfer belt. The volume resistance of this transfer belt was 2.5×10¹⁰Ω·cm. Further, the work function showed 5.37 eV, and the normalizedphotoelectron yield showed 6.90.

Production Example of Organic Photoreceptor (OPC1)

A coating solution obtained by dissolving and dispersing 6 parts byweight of an alcohol-soluble nylon resin (manufactured by TorayIndustries, Inc., CM8000) and 4 parts by weight of fineaminosilane-treated titanium oxide particles in 100 parts by weight ofmethanol was applied onto a conductive support of an aluminum pipehaving a diameter of 85.5 mm by ring coating, and dried at a temperatureof 100 ° C. for 40 minutes to form an undercoat layer having a thicknessof 1.5 to 2 μm.

A liquid dispersion obtained by dispersing 1 part by weight ofoxytitanium phthalocyanine as a charge generating agent and 1 part byweight of a butyral resin (manufactured by Sekisui Chemical Co., Ltd.,BX-1) in 100 parts by weight of dichloroethane for 8 hours in a sandmill using glass beads having a diameter of 1 mm was applied onto thisundercoat layer by ring coating, and dried at 80° C. for 20 minutes toform a charge generating layer having a thickness of 0.3 μm.

Onto the charge generating layer, a solution obtained by dissolving 40parts by weight of a charge transporting material of a styryl compoundrepresented by the following formula (I) and 60 parts by weight of apolycarbonate resin (manufactured by Teijin Chemicals Ltd., Panlite TS)in 400 parts by weight of toluene was applied by dip coating so as togive a dry thickness of 22 μm, and dried to form a charge transportinglayer, thus preparing an organic photoreceptor (1) having aphotosensitive layer comprising two layers of the charge generatinglayer and the charge transporting layer.

The resulting organic photoreceptor (1) was partially cut to form a testpiece, and the work function thereof was measured using a surfaceanalyzer (manufactured by Riken Keiki Co., Ltd., AC-2) at a dose ofirradiating light of 500 nW. As a result, it showed 5.48 eV.

Production Example of Organic Photoreceptor (OPC2)

An organic photoreceptor (OPC2) was prepared in the same manner as withthe production example of organic photoreceptor (OPC1) with theexception that a seamless nickel electroformed pipe having a thicknessof 40 μm and a diameter of 85.5 mm was used as the conductive support,and that the charge transporting material was changed to a distyrylcompound of the following formula (II). The work function of thisorganic photoreceptor was measured similarly. As a result, it was 5.50eV.

Production Example of Organic Photoreceptor (OPC3)

An organic photoreceptor was prepared in the same manner as with theproduction example of organic photoreceptor (OPC1) with the exceptionthat an aluminum pipe having a diameter of 30 mm was used as theconductive support. The work function of the resulting organicphotoreceptor was measured similarly. As a result, it was 5.48 eV.

Preparation of Surface-Modified Silica Particles

A hundred grams of silica particles (specific surface area: 130 m²/g)produced by a vapor-phase process were dispersed in 2000 ml of water,and the resulting liquid dispersion was heated to 70° C. Then, 250 ml ofan aqueous solution of titanium sulfate having a concentration of 100g/l as TiO₂ and a 5 N aqueous solution of sodium hydroxide wereconcurrently added dropwise so as to give a pH of 6.0. After thetermination of dropping, the solution was cooled to 40° C., and the pHwas adjusted to 4.0. Subsequently, 25 g of n-hexyltrimethoxysilane wasadded. After the solution was kept with stirring for 4 hours, a 2 Naqueous solution of sodium hydroxide was added to adjust the pH to 6.5,and the solution was further kept with stirring for 2 hours. Then, afterfiltration and water washing, the resulting filtered product was dried,and finely pulverized with a pulverizing mill to obtain silica particlessurface modified with an oxide. The resulting particles have a specificsurface area of 88.4 m²/g, and a degree of hydrophobicity of 62.5%.

The degree of hydrophobicity was measured by the following method.Aqueous methanol solutions different in methanol concentration wereprepared, and 10 ml of each aqueous methanol solution was poured into a25-ml test tube with a ground-in stopper. Then, 10 mg of a sample to bemeasured was put therein, and the methanol concentration (% by weight)at which sedimentation started was visually determined. The methanolconcentration thus determined was indicated as the degree ofhydrophobicity.

Further, the surface-modified silica particles had a mean primaryparticle size of about 18 nm. The work function thereof was measuredsimilarly. As a result, it was 5.38 eV.

Production of Combined Oxide Particles

FIG. 5 shows a burner apparatus for producing combined oxide particles.Referring to FIG. 5, the reference numeral 501 designates a combustionchamber, 502 designates a double-jacketed tube, 503 designates anannular diaphragm, 504 designates an inner tube, 505 designates an outertube, and 506 designates a water-cooled flame tube. The double-jacketedtube 502 projects in the combustion chamber 501, and a hot mixed vaporof 200° C. obtained by mixing 1.4 Nm³/h of hydrogen, 5.5 Nm³/h of airand 1.30 kg/h of previously evaporated gaseous SiCl₄ is introduced fromthe inner tube 504 of the double-jacketed tube 502. Then, gaseous AlCl₃previously evaporated at 300° C. is additionally supplied to this hotmixed vapor at a rate of 2.34 kg/h, and introduced into the flame tube.At the same time, 12 Nm³/h of air is additionally supplied to allow thevapor to burn. In this case, air is introduced into the combustionchamber, and air is additionally introduced from the annular diaphragm503. In the flame, water produced and a chloride are allowed to rapidlyreact with each other to form combined oxide particles. After havingpassed through the flame tube, a powder produced is separated using afilter or a cyclone, and a hydrochloric acid component adhered to thepowder is removed. The resulting combined oxide particles comprise 65%by weight of Al₂O₃ and 35% by weight of SiO₂, and have a mean primaryparticle size of 14 nm, a specific surface area by the BET method of 74m²/g and a volume resistance of 10¹² Ω·cm. The resulting combined oxideparticles were subjected to hydrophobic treatment with dimethylsilane(DMS). The work function of the resulting combined oxide particles wasmeasured similarly. As a result, the combined oxide particles showed twokinds of work functions, 5.18 eV and 5.61 eV.

Experimental Example 1

Hydrophobic fine silica particles (specific surface area by the BETmethod: 300 m²/g, mean primary particle size: 7 nm) treated withhexamethyldisilazane was added to and mixed with toner mother particles2 obtained above in an amount of 0.1% by weight, and then, externaladditives shown in Table 1 described below were each added to theresulting fine silica particle-coated toner in an amount of 0.2% byweight to prepare five kinds of magenta toners 2-1 to 2-5, respectively.

Measurement of the Rate of Liberation of Various External Additives fromToner Mother Particles

The rate of liberation of the various external additives shown in Table1 described below was determined using a particle analyzer (manufacturedby Yokogawa Electric Corporation, PT1000). The results thereof are shownin Table 2. The rate of liberation as used herein is calculated from thenumber of elements detected by the measurement with the particleanalyzer, and defined by the following equation:Number rate of liberation=(detected number of librated additive/alldetected number of additive)×100

TABLE 1 Specific Surface Area by BET Mean Primary Toner Method ParticleSize No. External Additive (m²/g) (nm) 2-1 Hydrophobic silica¹⁾ ca. 300ca. 7 2-2 Hydrophilic alumina ca. 100 ca. 13 2-3 Hydrophobic 135 ca. 20titanium oxide²⁾ (titania) 2-4 Combined oxide 100-110 ca. 17 particles³⁾2-5 Surface-modified 88.4 ca. 18 silica particles⁴⁾

As for the external additives (hydrophobilizing agents) in Table 1, 1)is hexamethylenedisilazane, 2) is n-butyltrimethoxysilane, 3) isdimethylsilane, and 4) is n-hexyltrimethoxysilane. The combined oxideparticles of 3) and the surface-modified silica particles of 4) are onesprepared above. TABLE 2 Number Rate of Liberation (%) Toner No. Si Al Ti2-1 0.63 — — 2-2 0.21 0.07 — 2-3 0.25 — 1.02 2-4 0.20 0.07 — 2-5 0.13 —0.12

As apparent from Tables 1 and 2, in the number rate of liberation of theexternal additives from the toner mother particles, it is found that thetoner (2-4) containing the combined oxide particles can restrain therate of Si liberation to a low level, compared to the toner (2-1)containing the fine silica particles. Compared to the toner (2-2)containing alumina, the toner (2-4) has a rate of Si liberationequivalent to or less than that of the toner (2-2) and a rate of Alliberation equivalent to that of the toner (2-2). Further, a comparisonbetween the toner (2-3) containing titania and the toner (2-5)containing the silica particles surface modified with titania revealsthat the latter is lower in both the rate of Si liberation and the rateof Ti liberation of the external additive, which shows that thesurface-modified silica particles strongly adhere to the toner motherparticles.

Image Formation Test and Measurement of Charge Characteristics

Hexamethyldisilazane-treated hydrophobic fine silica particles S (meanprimary particle size: about 7 nm) and hexamethyldisilazane-treatedhydrophobic fine silica particles L (mean primary particle size: about40 nm) were added to and mixed with the toner mother particles 1obtained above, at addition ratios shown in Table 3. Then, additivesother than silica described in Table 3 were added to and mixed with theresulting fine silica particle-coated toners at addition ratios (pertoner mother particles, % by weight) described in Table 3 to prepareseven kinds of cyan toners 1-1 to 1-7. The results of an image formationtest using each cyan toner and the results of charge characteristics areshown in Tables 4 and 5.

The image formation test was carried out using the 4-cycle color printershown in FIG. 3 which was equipped with organic photoreceptor (OPC1),the developing rollers, the regulating blades and transfer belt 1described above. The peripheral speed of the organic photoreceptor wasset to 180 mm/s, the ratio of the peripheral speed of the developingrollers to that of the photoreceptor was set to 1.3, and the differencein peripheral speed between the organic photoreceptor and theintermediate transfer belt was set so that the transfer belt becomes 3%faster than the organic photoreceptor. In a preliminary test, it wasconfirmed that exceeding 3% resulted in the occurrence of dust on atransferred image. Accordingly, the upper limit was set to 3%. Theregulating conditions of the above-mentioned toner regulating blade wereadjusted so that the amount of the toner transferred onto the developingroller reached 0.38 mg/cm². Each of the toners 1-1 to 1-7 was set in thecyan developing device, and the image formation was conducted by thenon-contact developing process (dark potential: −600 V, light potential:−80 V, DC developing bias: −200 V, AC bias: 1.4 kV, AC frequency: 2.5kHz) in which the developing gap was set to 210 μm. The image formationwas conducted on two sheet of a solid print and a 5% manuscript.

As for the charge characteristics, the charge characteristics of thetoner on the developing roller was measured with a charge distributionmeasuring device (E-SPART Analyzer Type EST-3) manufactured by HosokawaMicron Corporation, and the results thereof are shown in Table 4.Further, the solid image density (solid OD Value) in the imageformation, the fog density (fog OD Value) on the organic photoreceptorand the OD value of the reverse transfer toner transferred onto thephotoreceptor are shown in FIG. 5.

The solid OD value was determined by printing a solid image, andmeasuring the density of the image after fixing with a reflectiondensitometer (manufactured by a X-Rite, Inc., X-Rite 404).

Further, the fog density of a non-image area on the organicphotoreceptor (fog OD Value) was determined by a tape transfer method.In addition, the density of the so-called “reverse transfer toner”(Reverse Transfer Toner OD Value) which returned onto the organicphotoreceptor after the formation of the solid image was similarlydetermined by the tape transfer method.

The tape transfer method is a technique comprising attaching an adhesivetape (manufactured by Sumitomo 3M Ltd., Mending Tape 801-1-18) to thetoner on the organic photoreceptor, subsequently attaching the tape ontoa white paper, measuring the density from above the tape with areflection densitometer (manufactured by a X-Rite, Inc., X-Rite 404),and subtracting the density of an area to which no toner was transferredand only the tape was attached, from this measured value, therebydetermining the reflection density value. TABLE 3 Toner CompoundingRatio No. Combination of Additives (% by weight) 1-1 Silica S/SilicaL/alumina/titania/StMg S0.5/L0.3/0.2/0.5/0.2 1-2 Silica S/SilicaL/AlSi/TiSi/StMg S0.4/L0.3/0/0.5/0.2 1-3 Silica S/SilicaL/AlSi/TiSi/StMg S0.4/L0.3/0.01/0.2/0.2 1-4 Silica S/SilicaL/AlSi/TiSi/StMg S0.4/L0.3/0.05/0.1/0.2 1-5 Silica S/SilicaL/AlSi/TiSi/StMg S0.4/L0.3/0.1/0.05/0.2 1-6 Silica S/SilicaL/AlSi/TiSi/StMg S0.4/L0.3/0.2/0.01/0.2 1-7 Silica S/SilicaL/AlSi/TiSi/StMg S0.5/L0.3/0.5/0/0.2

In Table 3, toner 1-1 indicates to add hydrophilic silica (S) having amean primary particle size of 7 nm and hydrophilic silica (L) having amean primary particle size of 40 nm in amounts of 0.5% by weight and0.3% by weight, respectively, based on the toner mother particles, andthen, to add alumina described in Table 1, titania described in Table 1and magnesium stearate (StMg, manufactured by Kanto Kagaku, workfunction: 5.57 eV) were in amounts of 0.2% by weight, 0.5% by weight and0.2% by weight, respectively, based on the toner mother particles. Thesame is equally true for the toners 1-2 to 1-7. TABLE 4 Amount ofPositively Toner Average Amount Charged Toner No. of Charge (μc/g)(number %) 1-1 −15.14 5.7 1-2 −19.37 3.1 1-3 −17.53 2.9 1-4 −16.66 2.11-5 −16.11 1.3 1-6 −18.71 1.5 1-7 −20.33 4.1

TABLE 5 Toner Solid OD Reverse Transfer Toner No. Value Fog OD Value ODValue 1-1 1.37 0.04 0.01 1-2 1.25 0.01 or less 0.01 or less 1-3 1.300.01 or less 0.01 or less 1-4 1.32 0.01 or less 0.01 or less 1-5 1.340.01 or less 0.01 or less 1-6 1.27 0.01 or less 0.01 or less 1-7 1.220.03 0.01

The results shown in Tables 4 and 5 reveal that the toners (1-3 to 1-6)of the present invention make it possible to obtain desired chargecharacteristics and to improve image qualities (the prevention offogging and the reverse transfer toner, and ensuring of image density),even when they are used in smaller amounts than the toner (1-1). Theamount of the toner used equivalent to or less than the conventionalamount used can provide the equivalent or higher effect, which causes adecrease in the amount of external additives used, resulting in improvedfixability. Toner 1-2 shows that no addition of combined oxide particlesresults in a reduction in image density, and toner 1-7 shows that noaddition of surface-modified silica particles poses a problem withrespect to the prevention of fogging and the reverse transfer toner, andensuring of image density.

Experimental Example 2

Hydrophobic fine silica particles S (mean primary particle size: 12 nm)treated with hexamethylsilazane and hydrophobic fine silica particles L(mean primary particle size: 40 nm) similarly subjected to hydrophobictreatment was added to and mixed with toner mother particles 5 preparedabove in amounts of 0.5% by weight and 0.5% by weight, respectively.Then, additives other than silica shown in Table 6 were added to andmixed with the resulting fine silica particle-coated toners at additionratios (% by weight) described in Table 6 to prepare seven kinds of cyantoners 5-1 to 5-7. The evaluation results of image characteristics by animage formation test using each cyan toner are shown in Tables 7 and 8.TABLE 6 Toner Compounding Ratio No. Combination of Additives (% byweight) 5-1 Silica S/Silica L/alumina/titania/StCa S0.5/L0.5/0.2/0.5/0.25-2 Silica S/Silica L/AlSi/TiSi/StCa S0.4/L0.5/0/0.5/0.2 5-3 SilicaS/Silica L/AlSi/TiSi/StCa S0.4/L0.5/0.01/0.2/0.2 5-4 Silica S/SilicaL/AlSi/TiSi/StCa S0.4/L0.5/0.05/0.1/0.2 5-5 Silica S/SilicaL/AlSi/TiSi/StCa S0.4/L0.5/0.1/0.05/0.2 5-6 Silica S/SilicaL/AlSi/TiSi/StCa S0.4/L0.5/0.2/0.01/0.2 5-7 Silica S/SilicaL/AlSi/TiSi/StCa S0.5/L0.5/0.5/0/0.2

The meaning in Table 6 is the same as defined in Table 3, and StCa iscalcium stearate (manufactured by Kanto Kagaku, work function: 5.49 eV).TABLE 7 Amount of Positively Toner Average Amount of Charge ChargedToner No. (μc/g) (number %) 5-1 −14.92 7.3 5-2 −18.59 3.4 5-3 −17.18 3.35-4 −16.09 3.0 5-5 −15.81 2.8 5-6 −19.01 2.2 5-7 −21.10 5.9

TABLE 8 Toner Solid OD Reverse Transfer Toner No. Value Fog OD Value ODValue 5-1 1.37 0.04 0.01 5-2 1.25 0.01 or less 0.01 or less 5-3 1.300.01 or less 0.01 or less 5-4 1.32 0.01 or less 0.01 or less 5-5 1.340.01 or less 0.01 or less 5-6 1.27 0.01 or less 0.01 or less 5-7 1.220.03 0.01

The results shown in Tables 7 and 8 reveal that the toners (5-3 to 5-6)of the present invention make it possible to obtain desired chargecharacteristics and to improve image qualities (the prevention offogging and the reverse transfer toner, and ensuring of image density),even when they are used in smaller amounts than the toner (5-1). Theamount of the toner used equivalent to or less than the conventionalamount used can provide the equivalent or higher effect, which causes adecrease in the amount of external additives used, resulting in improvedfixability.

Experimental Example 3

The external additives were added to and mixed with toner motherparticles 2, toner mother particles 3 and toner mother particles 4 inthe same manner as with cyan toner 1-5 of Experimental Example 1 toprepare a magenta toner, a yellow toner and a black toner, respectively.

Then, using a 4-cycle color printer of an intermediate transfer mediumprocess shown in FIG. 3 in which an elastic photoreceptor of organicphotoreceptor (OPC2) was used and which was equipped with the developingrollers, the regulating blades and intermediate transfer belt 1, eachdeveloping unit was filled with each toner obtained above, and an imageformation test was carried out by a contact single-component developingprocess. As for the setting order of the developing devices, a firstdeveloping device filled with the magenta toner, a second developingdevice filled with the yellow toner, a third developing device filledwith the cyan toner and a fourth developing device filled with the blacktoner were arranged in this order from the upstream.

In the image formation, the peripheral speed of the organicphotoreceptor is set to 180 mm/s, the ratio of the peripheral speed ofthe developing rollers to that of the photoreceptor is set to 2, and thedifference in peripheral speed between the organic photoreceptor and theintermediate transfer belt is set so that the transfer belt becomes 3%faster than the organic photoreceptor. As for image formationconditions, the dark potential of the photoreceptor was set to −600 V,the light potential was set to −60 V, the developing bias was set to−200 V, and the developing rollers and supply rollers were set to thesame potential. Then, a constant voltage power supply was used as a highvoltage power supply of a primary transfer portion, and the transfervoltage was set to +450 V. A constant current power supply was used as ahigh voltage power supply of a secondary transfer portion, and thetransfer current was controlled to 16 μA. Under these conditions, acharacter manuscript of an A-4 size corresponding a 5% color manuscriptfor each color was continuously printed on 10,000 sheets of paper, andthe states on the photoreceptor and around the drum were checked. As aresult, the occurrence of fogging and the reverse transfer toner wasscarcely observed, and no scattering of the toner was observed, whichshowed stable toner charge characteristics. Then, when the amount of thetoners collected by cleaning the photoreceptor and the intermediatetransfer belt was measured, it was about 10 g, which corresponds toabout {fraction (1/30)} of the expected amount. This was the result ofbeing capable of improving the toner transfer efficiency and inhibitingthe occurrence of a fogging toner and the reverse transfer toner to theutmost.

Experimental Example 4

The external addition treatment was conducted to toner mother particles6, toner mother particles 7 and toner mother particles 8 in the samemanner as with cyan toner 5-5 obtained in Experimental Example 2 toobtain a magenta toner, a yellow toner and a black toner, respectively.

Then, each toner was loaded in each color developing cartridge of thefull color printer of the tandem system shown in FIG. 4, and the imageformation test was carried out by the non-contact single-componentdeveloping process. Organic photoreceptor (OPC3) was used as the organicphotoreceptor, the developing rollers and the regulating blades were thesame as described above, and a belt produced in accordance with theproduction example of transfer belt 1 was used as the intermediatetransfer belt.

As for the setting order of the developing devices, a first developingdevice filled with the black toner, a second developing device filledwith the yellow toner, a third developing device filled with the magentatoner and a fourth developing device filled with the cyan toner werearranged in this order from the upstream.

In the image formation, the AC superimposed on a DC developing bias of−200 V is applied under the conditions of a frequency of 2.5 kHz and aP-P voltage of 1,400 V, and the conditions of a primary transfer portionand a secondary transfer portion are set similarly to ExperimentalExample 3. Then, a character manuscript of an A-4 size corresponding a5% color manuscript for each color was continuously printed on 10,000sheets of paper, and the states on the photoreceptor and around the drumwere checked. As a result, the occurrence of fogging and the reversetransfer toner was scarcely observed, and no scattering of the toner wasobserved, which showed stable toner charge characteristics.

Then, when the amount of the toners collected by cleaning thephotoreceptor and the intermediate transfer belt was measured, it wasabout 23 g, which corresponds to about {fraction (1/13)} of the expectedamount. This was the result of being capable of improving the tonertransfer efficiency and inhibiting the occurrence of a fogging toner andthe reverse transfer toner to the utmost.

The surface-modified silica particle which is the external additive tothe negatively chargeable toner of the present invention has a negativefrictional charge site based on the silica component, and a positivefrictional charge site. Further, the silica component constituting abase particle easily adheres to the surface of the toner motherparticle. It is therefore conceivable that the surface-modified silicaparticle becomes difficult to liberate from the toner mother particle,compared to the case of addition of the conventional external additiveparticles such as titania and alumina, which makes it possible to impartstable charge characteristics in continuous printing for a long periodof time.

Further, as the external additives, the surface-modified silicaparticles and the combined oxide particles are added and mixed togetherwith the silica particles different in particle size, thereby being ablenot only to impart fluidity, but also to stabilize charge, preventfilming and prevent the occurrence of fogging and the reverse transfertoner to give stable full color print quality, even when they are addedin remarkably small amounts compared to the amount of the conventionalexternal additive particles such as titania and alumina added. Further,the fixability of the toner is not lowered. In particular, when they areused as the external additives in the toner produced by thepolymerization method, the amount thereof used can be decreased comparedto the conventional combined system of silica, titania and alumina, andthe fixability is not lowered.

Further, regardless of whether the toner is the toner obtained by thepulverization method or the toner produced by the polymerization method,a reduction in toner particle size requires to increase the amount ofsilica added. As a result, the amount of charge of the toner becomes toolarge at an early stage, the external additive is embedded or scatteredwith the progress of printing to decrease the effective surface amountof the external additive, which causes the problem of decreasing theamount of charge of the toner. In addition, the toner consumption tendsto increase because of fluctuations in image density and an increase inthe amount of fogging. It has been therefore difficult to use as thetoner. However, the negatively chargeable toner having the stable amountof charge throughout a printing period can be obtained by adding andmixing the surface-modified silica particles and the combined oxideparticles together with the silica particles different in particle size,as the external additives, to the negatively chargeable toner.

While the present invention has been described in detail and withreference to specific embodiments thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing the spirit and scope thereof.

The present application is based on Japanese Patent Application No.2003-197599 filed on Jul. 16, 2003, and the contents thereof areincorporated herein by reference.

1. A negatively chargeable toner comprising: resin particles containing a colorant; and an external additive which coats the surface of the resin particles, wherein the external additive comprises; first silica particles having a number mean primary particle size of 5 to 20 nm and second silica particles having a number mean primary particle size of 30 to 50 nm; surface-modified silica particles which are surface modified by wet treatment using an oxide or hydroxide of at least one metal selected from the group consisting of titanium, tin, zirconium and aluminum, and are further subjected to hydrophobic treatment; and aluminum oxide-silicon dioxide composite oxide particles obtained by flame hydrolysis and hydrophobic treatment.
 2. The negatively chargeable toner according to claim 1, wherein the total amount of the first and second silica particles are 0.5 to 1.5% by weight based on the weight of the resin particles, wherein the weight ratio of the first silica particles to the second silica particles is from 5/1 to 1/5.
 3. The negatively chargeable toner according to claim 1, wherein the amount of the surface-modified silica particles is 0.005 to 0.5% by weight based on the weight of the resin particles, wherein the amount of the aluminum oxide-silicon dioxide composite oxide particles is 0.005 to 0.5% by weight based on the weight of the resin particles, wherein the total amount of the surface-modified silica particles and the aluminum oxide-silicon dioxide composite oxide particles is from 0.01 to 1% by weight based on the weight of the resin particles.
 4. The negatively chargeable toner according to claim 1, wherein the amount of the external additive is 0.51 to 2.5% by weight based on the weight of the resin particles.
 5. The negatively chargeable toner according to claim 1, which is produced through a polymerization method.
 6. The negatively chargeable toner according to claim 1, wherein the negatively chargeable toner has a sphericity of 0.94 or more.
 7. The negatively chargeable toner according to claim 1, wherein the negatively chargeable toner has a number mean particle size of 9 μm or less.
 8. A full color toner comprising the negatively chargeable toner according to claim
 1. 9. A process for producing a negatively chargeable toner comprising the steps of: adding, to resin particles containing a colorant, first silica particles having a number mean primary particle size of 5 to 20 nm and second silica particles having a number mean primary particle size of 30 to 50 nm; and further adding thereto surface-modified silica particles which are surface modified by wet treatment using an oxide or hydroxide of at least one metal selected from the group consisting of titanium, tin, zirconium and aluminum, and are further subjected to hydrophobic treatment, and aluminum oxide-silicon dioxide composite oxide particles obtained by flame hydrolysis and hydrophobic treatment.
 10. A full color image forming apparatus comprising: a photoreceptor on which toner images are to be formed; an intermediate transfer medium for transferring the toner images formed on the photoreceptor to a recording medium; and a toner for forming the toner images comprising: a negatively chargeable toner comprising: resin particles containing a colorant; and an external additive which coats the surface of the resin particles, wherein the external additive comprises; first silica particles having a number mean primary particle size of 5 to 20 nm and second silica particles having a number mean primary particle size of 30 to 50 nm; surface-modified silica particles which are surface modified by wet treatment using an oxide or hydroxide of at least one metal selected from the group consisting of titanium, tin, zirconium and aluminum, and are further subjected to hydrophobic treatment; and aluminum oxide-silicon dioxide composite oxide particles obtained by flame hydrolysis and hydrophobic treatment.
 11. The full color image forming apparatus according to claim 10, wherein the photoreceptor is a negatively chargeable organic photoreceptor.
 12. The full color image forming apparatus according to claim 10, wherein the intermediate transfer medium is a belt.
 13. The full color image forming apparatus according to claim 10, further comprising a image developing device, wherein the photoreceptor and the image developing device are integrated to form a process cartridge, wherein the process cartridge is detachably mounted on the image forming apparatus.
 14. The full color image forming apparatus according to claim 10, wherein the ratio of peripheral velocity of the photoreceptor to the intermediate transfer medium is from 0.95 to 1.05.
 15. The negatively chargeable toner according to claim 1, wherein the first and second silica particles are subjected to hydrophobic treatment.
 16. The process for producing a negatively chargeable toner according to claim 9, wherein the first and second silica particles are subjected to hydrophobic treatment.
 17. The full color image forming apparatus according to claim 10, wherein the first and second silica particles are subjected to hydrophobic treatment.
 18. The negatively chargeable toner according to claim 1, wherein the external additive further comprising a metal soap particle.
 19. The process for producing a negatively chargeable toner according to claim 9, further comprising a step of adding a metal soap particle to the resin particles.
 20. The full color image forming apparatus according to claim 10, wherein the external additive further comprising a metal soap particle.
 21. The negatively chargeable toner according to claim 3, wherein the weight ratio of the surface-modified silica particles to the aluminum oxide-silicon dioxide composite oxide particles is from 2/50 to 50/2.
 22. The full color image forming apparatus according to claim 10, wherein the work function of the negatively chargeable toner is higher than the work function of the surface of the photoreceptor. 